Hotspot Scorched Midwest, Leaving Legacy of Earthquakes, Rare Rocks

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A seismic speed trap that stretches from Missouri to Virginia
suggests a hotspot scorched the Midwest during the Mesozoic Era,
a new study finds.

Hotspots are scalding plumes of hot rock rising toward Earth's
surface from the mantle, the layer that sits under Earth's crust.
Though tectonic plates constantly shift,
hotspots are homebodies, stuck in pretty much the same spot
for their entire lives, scientists think.

When continents trundle across hotspots, the plumes burn their
bottoms, like a cold plate passed over a flame. For the first
time, scientists have spied one of these warm trails under North
America's midriff, according to a study published today (Sept.
15) in the journal Nature Geoscience.

The study authors think the hot zone lines up with rare rocks
called
kimberlites in Kansas and Kentucky. The deep-seated heating
also accounts for thinning of the crust and intrusions of magma
along the New Madrid rift during the Mesozoic, the researchers
said. The
New Madrid rift is a seismic zone in the central Mississippi
Valley that split open in the Early Cambrian period and
reactivated during the Mesozoic. During the winter of 1811 to
1812, three powerful earthquakes in the rift shook New Madrid,
Mo., and rattled buildings as far away as New York and
Washington, D.C.

"I think this is all very exciting because it means that the
continent is not as boring as everybody thought it was," said
study co-author Don Helmberger, a Caltech seismologist. "I grew
up in the Midwest and thought this was the worst place in the
world [for geology]."

Connecting the dots

Because kimberlites speckle the Midwest, for decades geologists
have suggested North America glided over a hotspot as it marched
westward for the past 100 million years. Though the Kansas and
Kentucky kimberlites are diamond-free, in Canada and Africa,
diamond-bearing kimberlites bubbled up above hotspots. The magma
that creates kimberlites travels hundreds of kilometers in mere
days, exploding violently once it reaches the surface. It's as if
the Earth farts diamonds. [ Sinister
Sparkle Gallery: 13 Mysterious & Cursed Gemstones ]

Until now, no one had seen telltale signs of a hotspot beneath
the K-state kimberlites.

"People have always thought these were caused by a hotspot, but
no one had proof," Helmberger said.

Finally, in 2011, the combination of a rare East Coast earthquake
and the placement of the USArray, a dense
traveling seismometer network, gave researchers a clear look
under the Midwest. "You have to have some luck to get a foothold
on the interesting stuff, you know," Helmberger told
LiveScience's OurAmazingPlanet.

Slow and low

The massive array of more than 500 seismometers, instruments that
measure earthquake waves, spied an unusually slow zone beneath
North America during the
2011 Virginia earthquake. This seismic "low-velocity" lane
cuts west to east from Missouri to Virginia, then jogs north to
Massachusetts and continues offshore. The corridor is 25 miles
(40 kilometers) thick, and lies 75 miles to 100 miles (120 to 160
km) below the surface, said Risheng Chu, lead study author and a
geophysicist with the Institute of Geodesy and Geophysics at the
Chinese Academy of Sciences in Beijing. The zone also appears in
other earthquakes, Chu said in an email interview.

A computer model of the hotspot's heating suggests it could have
created the slow zone, the researchers report. Higher
temperatures in rocks can hinder seismic waves. For example, the
model's predicted temperatures match the real-world seismic
slowing one might see if a hotspot passed under Kentucky 75
million years ago, when the state's kimberlites punched through
the crust, the study finds.

"This is a hidden track, meaning it's way down there, but it's
still a couple hundred degrees hotter than it normally would be,"
Helmberger said.

But other than the kimberlites and the New Madrid rift getting
hotter and thinner in the Mesozoic, Chu and his colleagues found
no other geologic connections to the hotspot. But geophysicist
Cindy Ebinger, who wasn't involved in the study, notes that
Virginia has a couple volcanoes in the right place at the right
time, and there are kimberlites in Pennsylvania and New York.
[ In
Images: How North America Grew As a Continent ]

Ebinger said the team's evidence for seismic velocity differences
under North America was "unquestionable," but the link to surface
geology was more tenuous.

"The age part starts with a guess based on a few pieces of
circumstantial evidence," said Ebinger, a professor at the
University of Rochester in New York. "Maybe there are other
features and other tracks they could connect to. I think this
will motivate new and better studies that will take it out of the
speculative range."

East Coast flow

Chu and his colleagues believe the hot spot now sits under the
flat, featureless Sohm Plain, north of a chain of unrelated
volcanic islands called the New England Seamounts. (In fact, the
seamounts formed about 110 to 80 million years ago over the Great
Meteor
mantle plume ). They hope to confirm their suspicions with the
mobile seismometer network, which shifted east of the
Appalachians this year, Helmberger said.

"This is a novel interpretation that will be testable as
EarthScope moves eastward," said Ebinger, referring to the
traveling seismometer network. Ebinger said she expects the
network will reveal that the East Coast isn't as simple as
textbooks predict.

"It's exciting to see that new discoveries are being made about
the structure and perhaps even the evolution of the continental
lithosphere of North America," she told LiveScience's
OurAmazingPlanet.